1. Field of the Invention
The present invention relates to liposomes and more particularly to antineoplastic agent-entrapping liposomes.
2. Related Arts
As having been widely recognized, liposomes are lipid vesicles to be prepared by suspending a polar lipid film in an aqueous solution. These liposomes have basically the same structure as cell membranes in the living body and thus have been widely employed as a model for studying biological membranes. Further, studies have been made towards the application of liposomes as a vehicle or carrier for a drug or enzyme into the living body.
In case where the antineoplastic agent is entrapped within liposomes, the following advantages are expected:
1. Protection of the encapsulated agent from various metabolic enzymes, PA1 2. Reduction of the essential toxicity of the antineoplastic agent, PA1 3. Sustentation of pharmacological effect due to slow release, PA1 4. Improvement in arrival to target tissues, and PA1 5. Selective uptake into cytoplasm or lysosomes. PA1 (a) The amount of agent entrapped in the liposomes is as large as possible; thus, the lipid dosage is at its minimum. PA1 (b) The accumulation of agent in the heart and kidney is low so as to suppress the manifestation of cardiotoxicity and nephrotoxicity as much as possible. PA1 (c) The applicable negatively charged lipids show a higher cost-performance to allow the mass production of antineoplastic agent-entrapping liposomes. PA1 (d) The accumulation of the antineoplastic agent entrapped in liposomes in the liver is similar to the level of free antineoplastic agent, because the administration of a relatively large amount of the agent may cause adverse toxicity to the liver, and PA1 (e) The concentration of agent is kept at a level high enough to attain a stable and sustaining manifestation of its pharmacological effect.
For example, adriamycin, one of the most active antineoplastic agents, is a glycosidic anthracycline antibiotic that is a fermentation product of the fungus, Streptomyces peucetius var. caesius. Adriamycin has a tetracycline ring structure with positively charged daunosamine attached by a glycosidic linkage. This drug is known to be useful for the treatment of leukemias and solid tumors; it attacks heart muscle, however, to cause irreversible injury. As a result, chronic cardiotoxicity has limited the clinical use of this drug for human.
Several workers have suggested an entrapment of adriamycin or its related anthracycline glycosides in negatively charged liposomes to reduce the cardiotoxicity and nephrotoxicity. In order to entrap adriamycin effectively, many kinds of negatively charged lipids have been utilized as the effective component of the liposomal membranes. Foressen et al. have shown that phosphatidylserine liposomes reduce the chronic cardiotoxicity. ["Proc. Natl. Acad. Sci. USA." 78, 1973-1877 (1981)], van Hoesel et al. have also observed reduction of the cardiotoxicity and nephrotoxicity when the drug in phosphatidylserine liposomes was administered ["Cancer Res." 44, 3698-3705 (1984)]. Rahman et al. demonstrated that chronic administration to beagle dogs of adriamycin entrapped in cardiolipin liposomes did not produce any cardiac abnormalities ["Cancer Res." 43, 5427-5432 (1983)]. Kojima et al. have shown that sulfatide liposomes gave the highest degree of adriamycin entrapment of all negatively charged lipids ["Biotechnol. Appl. Biochem". 8, 471-478 (1986)].
These negatively charged lipids as the effective component of the liposomal membranes are usually derived from natural materials, but the purification of these lipids is not easy; furthermore, complete synthesis of these lipids is complicated. As a result, these negatively charged lipids have proved to be expensive for clinical application of such liposomes; therefore, the mass production of antineoplastic agent-entrapping liposomes has been considered as unsuitable in terms of its manufacturing cost. On the other hand, instead of using these negatively charged lipids as components of liposomal membranes, synthetic anionic detergents may be adopted. Nevertheless, these detergents are not natural materials derived from the living body; and, for administration to man, safety requirements must be carefully established because most liposomes are primarily metabolized in the liver where they are taken up by the reticuloendothelial system. Accordingly, application of these detergents as a carrier for drugs has not been accepted in clinical field.
It would be desirable to satisfy the following requirements as an ideal liposome preparation that would provide an effectiveness, and suitable utilization of an antineoplastic agent such as adriamycin, daunorubicin or the like.